1. Field of the Invention
The present invention relates to a light beam scanning device having nonlinear drive current-light amount characteristics (I-L characteristics), a method of controlling the same, a storage medium, and an image forming apparatus including the light beam scanning device.
2. Description of the Related Art
It is known that in a general semiconductor laser diode used in an image forming apparatus and the like, the slope of light emission characteristics representative of a correspondence relationship between drive current and light amount (hereinafter referred to as the “I-L characteristics”) is non-linear, as shown in FIG. 17. Further, in such a laser diode, there has been proposed the following control technique for controlling a target light amount: A threshold current Ith of the laser diode and a slope η of the I-L characteristics are calculated based on light amounts (P1 and P2) at two points and drive currents (I1 and I2) associated therewith, and based on the results of the calculations, a drive current (I3) associated with a desired light amount (P3) is calculated and set (see e.g. Japanese Patent Laid-Open Publication No. H05-145154).
By the way, assuming that the I-L characteristics are non-linear as in the case of a surface emitting laser diode (VCSEL), as the difference between the light amounts (P1 and P2) at two points from which the slope of the I-L characteristics is calculated is larger, an error between the calculated slope and an actual slope becomes larger, as shown in FIG. 18. This produces a difference between the light amount (P3) as a target light amount and a light amount (P3′) actually controlled, so that the accuracy of the light amount control is degraded.
On the other hand, a technique has been proposed in which the light amount is controlled by storing the relationship between temperature and I-L characteristics in a memory, monitoring temperature to read out I-L characteristics associated therewith, and setting a drive current based on the read I-L characteristics (see e.g. Japanese Patent Laid-Open Publication No. 2002-100831). Further, a method as well has been proposed in Japanese Patent Laid-Open Publication No. 2002-100831, in which I-L characteristics stored in the memory are corrected by emitting light from a laser diode using a predetermined drive current, irradiating a photosensitive drum with the light, and measuring the surface potential of the photosensitive drum.
However, in general, the I-L characteristics of the laser diode are changed not only by temperature but also by aging, as shown in FIG. 19. Therefore, the method of storing I-L characteristics in a manner associated with temperature and performing the light amount control based on the stored I-L characteristics suffers from a problem that a change in the I-L characteristics caused by aging cannot be followed up, whereby the accuracy of the light amount control is lowered as aging advances.
Further, in the case of the method in which the photosensitive drum is irradiated using a plurality of light amounts, and the I-L characteristics of the laser diode are predicted and corrected based on the surface potentials of the photosensitive drum, it is difficult to know the characteristics of the laser diode alone because the control becomes complicated the photosensitive drum has a characteristic that the relationship between the amount of change in the surface potential and the amount of exposure is not linear, and so forth. This brings about the problem that when the optical correction of the light beam scanning device is performed, there occurs a large correction error.